1. Field of the Invention
The present invention is related to a device for controlling an amount of light transmitted from a light source of a video processor which functions as a lighting unit for an endoscope.
2. Background of the Invention
In an endoscope, light is transmitted from a light source using a light wave guide, such as an optical fiber cable, in order to illuminate an object to be observed. In order to adjust the brightness of the observed image, a device for controlling an amount of light transmitted from a light source to an incident surface of the light wave guide, is employed. In a conventional endoscope, the light amount controlling device has a light shield which is rotated about an axis by a stepping motor. The rotation of the light shield controls the amount of light from the light source that is incident on the incident surface of the optical fiber cable. With this type of light amount controlling device, the brightness of the observed image is detected periodically. Then, the position of the light shield is adjusted such that the brightness of the observed image is within an allowed brightness range.
Conventionally, the amount of light transmitted from the light source to the endoscope is controlled by applying the same number of pulses to the input of the stepping motor during each interrupt procedure (see FIG. 11A). Therefore, the stepping motor and the light shield are rotated by the same angular amount during each interrupt procedure. The process of detecting the brightness level, and driving the stepping motor to rotate the light shield is repeated until the detected brightness is again within the allowed brightness range.
However, in the conventional endoscope, since the number of driving pulses sent to the motor is constant during the execution of each interrupt procedure, if the number of pulses is set to a relatively small value, then the light shield will be moved slowly. This results in an increase in the response time of the light amount controlling device.
As shown in FIG. 11A, for example, each drive pulse rotates the motor 0.5 degrees, three drive pulses are sent during each interrupt. The interrupts are executed every 50 ms. Thus, in order to rotate the stepping motor 10 degrees, seven interruption procedures are required for a total time of 0.35 seconds. Further, since the number of pulses must be in multiples of three, the number of drive pulses cannot be 20, the optimum number, but must be 18 or 21. Therefore, the light shield cannot be moved to the optimum position.
In order to decrease the response time of the light amount controlling device, the number of driving pulses sent to the stepping motor can be increased. However, in this case, the light shield will be moved through a large angle of rotation and thus it may not be possible to adjust the amount of light such that the brightness level falls within the allowed brightness range. This will result in the control system becoming unstable with unwanted back-and-forth oscillations (hereinafter referred to as hunting) occurring.
Further, as different types of endoscopes have different allowed brightness ranges, different numbers of driving pulses are required in order to properly adjust the amount of light transmitted from the light source.
Furthermore, for one type of endoscope, if the sending of a predetermined number of pulses to the stepping motor does not cause hunting, in another endoscope, the predetermined number of pulses may not be sufficiently low, and hunting may occur. Therefore, since the light source may be connected with various types of endoscopes, the number of pulses may be set to the minimum number required for all types of endoscopes in order to avoid the hunting problem. As a result, the speed at which the brightness level can be adjusted for the types of endoscopes is reduced.